Motor control devices generally include a control circuit for controlling the speed and position of a motor, a rectifying circuit for converting AC power to DC power, and a drive circuit for supplying electric current to the drive motor. The drive circuit supplies power to the motor by switching its power transistors on and off.
In conventional motor control devices having an emergency stop function, when an emergency stop signal is received, the drive circuit stops outputting an on-off signal to the power transistors. As a result, the power transistors stop power to the motor so as to emergency-stop the motor.
In a robot control system or other equipment where such a motor control device is installed, an emergency stop can be activated as follows. First, the control circuit in such system or equipment determines whether an emergency stop signal has been received. Then, the control circuit controls the motor control device to stop transmitting a motion command to the motor if necessary, so as to emergency-stop the motor.
There has been proposed a technique for reducing the risk of failure of an emergency stop circuit in a motor control device (see, e.g., Patent Literature 1). Such a conventional emergency stop circuit includes two emergency stop signals. These emergency stop signals are inputted to respective CPUs (central processing units) which check the consistency between these signals. The emergency stop circuit stops a gate drive signal based on the checking result so as to emergency-stop the motor. Thus, the conventional emergency stop circuit has improved its safety performance by having two emergency stop signals.
In spite of having two paths of emergency stop signals, the conventional emergency stop circuit has only one shut-off function to shut off the motor. This means that when the shut-off function itself breaks down, the motor may not be successfully emergency-stopped.
Furthermore, the conventional emergency stop circuit requires a complex software process because the CPUs are disposed between a plurality of circuits for transmitting the emergency stop signals. Therefore, when the software has a problem, received emergency stop signals may not be processed properly.
Since the conventional emergency stop circuit stops power to the motor by the software process by the CPUs, the emergency stop signals are checked at time intervals determined by a software program. This causes a delay in responding to emergency situations by the time intervals.
Thus, if the emergency stop function in the conventional motor control device breaks down, this may cause the motor to continue to rotate against the user's intention. To avoid such a consequence, much safer motor control devices have been required.    Patent Literature 1: Japanese Patent Unexamined Publication No.2006-268130